Actually, event organizers peg the attendance at over 1,000,000:

World Congress of Families (WCF) hailed Sunday's march for traditional, natural marriage in Paris. WCF Managing Director Larry Jacobs commented: "Hundreds of thousands marched in Paris yesterday, in bitterly cold temperatures, to oppose plans by President Francois Hollande to force a bill legalizing so-called same-sex marriage through the parliament. Yesterday's march, supported by the French Catholic hierarchy, was an impassioned outpouring in defense of marriage, children, and the natural family. It demonstrates, once again, that the push for 'same-sex marriage' is driven by elites and rejected by the overwhelming majority of families worldwide."

...Jacobs concluded: "We congratulate the organizers of the Paris march and declare our solidarity with efforts to defend natural marriage in the United States, the United Kingdom, Australia and wherever such efforts are underway. Just as the campaign to undermine marriage . . . is international in scope, the defense of natural marriage must stretch across national boundaries to embrace families everywhere." (Breaking Christian News)